The Nonlinear
and Ultrafast Fiber Optics Laboratory specialize in nonlinear optics and
photonic crystal fibers, and their use for infrared frequency metrology. This
year we have made significant contributions to these fields.

My
publication list can be found here.
My past research can be found here.

Passively Mode-Locked
Thulium/Holmium Laser at 2 μm

We have demonstrated a
passively mode-locked Tm/Ho co-doped fiber laser that operates in both the solitonic
and stretched-pulse regime by controlling the intracavity net
dispersion.In the solitonic regime
the laser produces 1.24 ps pulses with 9 nm
spectral bandwidth.By adding a
positive dispersion fiber to the cavity the laser was able to operate in the
stretched-pulse regime with a bandwidth of 30 nm and duration of 450 fs.

We demonstrate for the first
time an optically pumped gas laser based on population inversion using a
hollow core photonic crystal fiber (HC-PCF). The HC-PCF filled with 12C2H2
gas is pumped with ~ 5 ns pulses at 1.52 μm
and lases at 3.12 μm and 3.16 μm in the mid-infrared spectral region. The maximum
measured laser pulse energy of ~ 6 nJ was obtained
at a gas pressure of 7 torr with a fiber with 20
dB/m loss near the lasing wavelengths. While the measured slope efficiencies
of this prototype did not exceed a few percent due mainly to linear losses of
the fiber at the laser wavelengths, 25% slope efficiency and pulse energies
of a few mJ are the predicted limits of this laser.
Simulations of the laser’s behavior agree qualitatively with experimental
observations.

A frequency comb generated
by a 167 MHz repetition frequency erbium-doped fiber ring laser using a
carbon nanotube saturable absorber is
phase-stabilized for the first time. Measurements of the in-loop phase
noise show an integrated phase error of 0.35 radians, which is a factor of
three larger than that of another fiber frequency comb based on a
figure-eight laser. For further investigation of stability, we heterodyned
the carbon nanotube laser comb with a 1532 nm CW laser stabilized to a ν1+ν3
overtone transition of an acetylene-filled kagome photonic crystal fiber
reference. These measurements resulted in an upper limit on the comb
stability of 1.2x10-11 in 1 s. The carbon nanotube laser
frequency comb offers much promise as a robust and inexpensive all-fiber
frequency comb with further potential for scaling to higher repetition
frequencies.

Saturated
absorption spectroscopy reveals the narrowest features so far in
molecular-gas-filled hollow-core photonic crystal fiber. The 48 - 68 μm core diameter of the kagome-structured fiber used
here allows for 8 MHz full-width half-maximum sub-Doppler features, and its
wavelength-insensitive transmission is suitable for high-accuracy frequency
measurements. A fiber laser is locked to the 12C2H2n1+n3P(13)
transition inside kagome fiber, and compared with frequency combs based on
both a carbon nanotube fiber laser and a Cr:forsterite
laser, each of which are referenced to a GPS-disciplined Rb
oscillator. The absolute frequency of the measured line center agrees
with those measured in power build-up cavities to within 9.3 kHz (the 1
σ error bar). The fractional stability is less than 1.2´10-11 at 1 s averaging
time.

A
gas lasing medium offers advantages over solid-state materials for high power
laser applications that require high electrical-to-optical power
efficiency. Gas lasers offer higher power efficiency due to, in part,
the higher quantum efficiency of the gas. In addition, gas lasers have
superior heat management properties that facilitate demonstrations of
continuous wave powers in the megawatts. Unfortunately, the drawbacks
of a gas medium are in containment, efficiently pumping the lasing
transition, and the small gain per unit length. These drawbacks have lead to commercial systems that favor solid-state lasers,
which have tended to supplant gas lasers in many research and industrial
applications. A prime example is the replacement of bulky,
power-consuming argon ion lasers by small semiconductor-pumped solid-state
green lasers for many scientific and industrial applications.
Currently, solid state lasers offer a more compact and reliable laser at the
sacrifice of lower quantum efficiency.

We wish
to create a new class of lasers through the amalgamation of hollow-fiber and
optically-pumped-gas technologies. The new laser will have a molecular
gas lasing medium in a hollow fiber that will be optically pumped using a
fiber-coupled laser diode or a fiber laser. This novel laser will have
the advantages of quantum efficiency of a gas medium with laser cavity that
is compatible with fiber-coupled laser diodes and fiber components.
Applications for lasers that that exhibit efficient electrical-to-optical
power transfer are in laser ranging and missile defense. This program
will also help to develop new technology for possible industrial applications
such as precision machining and cutting. The results from this research
will help develop high-power gas lasers in the infrared atmospheric
transmission windows (3.5 to 4.1 mm).

A continuous-wave laser has
been stabilized to an acetylene transition inside kagome photonic crystal
fiber. Stability as measured with a carbon nanotube fiber laser frequency
comb to is better than 1x10-11 at 10 s.

Infrared frequency combs
based on mode-locked erbium-doped fiber lasers typically require an external
amplifier since the pulses directly from the laser have insufficient peak
power to generate an octave-spanning supercontinuum for self-referencing.
Here we implement a unique, all-fiber erbium-doped fiber amplifier that uses
hollow-core photonic bandgap fiber for pulse compression. Through a combination
of experiment and numerical simulations we have demonstrated temporal
compression in the hollow-core photonic bandgap fiber, thus increasing the
pulse’s peak power.

The difficulty of fusion
splicing hollow-core photonic bandgap fiber (PBGF) to conventional step index
single mode fiber (SMF) has severely limited the implementation of PBGFs. To
make PBGFs more functional we have developed a method for splicing a
hollow-core PBGF to a SMF using a commercial arc splicer. A repeatable,
robust, low-loss splice between the PBGF and SMF is demonstrated. By filling
one end of the PBGF spliced to SMF with acetylene gas and performing
saturation spectroscopy, we determine that this splice is useful for a PBGF
cell.

The frequency comb from a
prism-based Cr:forsterite
laser has been frequency stabilized using intracavity prism insertion and
pump power modulation. Absolute frequency measurements of a CW fiber laser
stabilized to the P(13) transition of acetylene demonstrate a fractional
instability of ~2×10-11 at a 1 second gate time, limited by a
commercial GPS disciplined rubidium oscillator. Additionally, absolute
frequency measurements made simultaneously using a second frequency comb
indicate relative instabilities of 3×10-12 for both combs for a 1
second gate time. Estimations of the carrier envelope offset frequency
linewidth based on relative intensity noise and the response dynamics of the
carrier envelope offset to pump power changes confirm the observed
linewidths.